The present invention relates to stable thixotropic gel fuel compositions wherein a solid combustible material is suspended in a liquid fuel along with a gelling agent and, optionally, an ash modifier, and a novel method of making them. The compositions are intended to be burned as fuels in standard type residential, commercial and industrial oil burners.
Recent events have underscored the need to conserve our natural resources, and more particularly, our dwindling supply of oil. One suggested manner of dealing with the energy problem has been to use more coal and solid fuels until more oil or alternate forms of energy can be discovered and commercially developed. The use of solid fuels could also reduce our dependence on foreign oil. However, most residential, commercial and industrial fuel users do not have equipment which can presently use solid fuels. Accordingly, the equipment, generally consisting of gas or oil furnaces, must either be replaced or converted to be able to burn the solid fuel.
The present invention comprises a method of making stable thixotropic gel fuel compositions and the compositions so produced which can be burned in standard types of oil furnaces, such as that described in J. J. Demeter et al., "Combustion of Coal-Oil Slurry in a 100 HP Firetube Boiler," U.S. Energy Research and Development Administration publication #PERC/RI-77/8, May 1977, with a minimal increase in cost. The Library of Congress Congressional Research Service has reported that the cost involved in converting a gas fired furnace or an oil fired furnace to one capable of burning presently known coal/oil mixtures would be about one-fourth of the cost of replacing a gas fired furnace or oil fired furnace with a solid fuel fired furnace. It is believed that the cost of conversion would be even less with the compositions of the present invention.
The method of the present invention basically involves the absorption of combustion enhancing material in porous solid carboniferous combustible material. This component is then suspended in a liquid fuel and a gelling agent to form a stable thixotropic gel fuel composition. The amount of heat energy derived from a given volume of compositions according to the present invention having a high solids content should be significantly more than the heat derived from the same volume of liquid fuel, apparently because the generally denser, solid combustible material is able to deliver more heat than the generally less dense liquid fuel. Thus, by substituting more plentiful solid combustible material for the less plentiful liquid fuel, more heat energy can be delivered while liquid fuel resources can be conserved. In addition, the combustion enhancing material which is absorbed in the porous solid combustible material significantly contributes to the efficient and essentially complete combustion of the composition.
The solid combustible material particles incorporated in the thixotropic gel fuel composition of the present invention will remain suspended indefinitely, overcoming the stability and settling problem of prior art compositions. There is no settling or agglomeration of the solid combustible material particles. Nor is there any need to resort to the prior art technique of ultra-fine grinding of the solid combustible material which was used to produce a suspension that was stable only for about one month. Thus, the solid combustible material intended for use in the present invention need only be ground in the customary manner of grinding the material when it is intended for combustion purposes.
The thixotropic gel compositions of the present invention do not require complex suspending agents, such as coal tar or surface-active materials, which interfere with the atomization process in the furnace or burner. Rather, the present fuels are formulated of particular gelling agents to form thixotropic gels which can immediately break apart upon injection in the furnace.
In addition, the thixotropic gel fuels made from the hereinafter described particulate gelling agents are far less sensitive to temperature thinning than prior art compositions. Thixotropic gelation provides gels having the advantage of solid-like immobility, similar to grease, during storage and transportation, but imparts to the compositions the ability to shear thin to apparent viscosities approximating those of the unmodified liquid fuel when pumped or atomized. Therefore, the thixotropic gel fuels of the present invention can be transported, delivered and used with the conventional equipment and methods used with liquid fuels, such as fuel oil.
Numerous attempts have been made in the past to provide a stable suspension of slurry of a solid combustible material in a liquid combustible material, but without much success. Problems of stability, transportation, storage, handling and use have yet to be overcome. The present invention overcomes these problems and is distinguishable from and patentable over prior art compositions, including those disclosed in the following patents, which are believed to be the most relevant prior art known to applicant.
U.S. Pat. No. 2,938,779 of Kolfenbach et al. discloses a jet fuel having three components: a liquid hydrocarbon fuel; particles of a combustible solid selected from the class consisting of aluminum, magnesium and carbon; and a carbon black of high structure index. It is disclosed that the jet fuel compositions generally possess a semi-fluid or gel-like structure in which the particles of combustible solid are uniformly suspended throughout the liquid hydrocarbon component by the structure black. It is stated that the fuels are characterized by a marked structural stability and uniformity over a wide range of temperatures and they possess flow characteristics that permit them to be transported by means of pumps, lines, etc. There is no mention in the patent, however, that the gels are thixotropic, nor is there any disclosure of the method of making them.
Moreover, there is no indication or suggestion in the patent that the jet fuel compositions may be used with standard type oil burners. Thus, a composition which is a suitable jet fuel may not be suitable for use in heating and for other commercial and industrial purposes. In addition, the carbon blacks disclosed in the patent may not be equivalent to the carbon blacks used in the compositions of the present invention. Thus, the difference in structure between carbon blacks may result in different properties for the compositions containing the carbon blacks.
U.S. Pat. No. 3,414,443 of Pheasant et al. discloses incendiary compositions useful in distress signals and for distress fires. The compositions comprise from 30 to 75 volume percent of finely divided solid fuels having a heat of combustion greater that 9.0 Kcal./ml. uniformly disposed in a gelled meltable solid fuel. The solid fuels comprise finely divided metal powders, such as boron, aluminum, zirconium, magnesium, lithium and metal hydrides. The meltable solid fuels include paraffin wax or lithium metal which are gelled while in the liquid state. Some of the gelling agents disclosed in this patent may be used to gel the composition according to the present invention.
It is clear that the incendiary compositions disclosed in this patent are not suitable for burning in oil burners. Problems relating to transportation, storage, delivery and use would result from the presence of the meltable solid fuels in the compositions. This patent does not disclose the method of making the gel composition.
U.S. Pat. No. 3,620,698 of Schlinger et al. discloses a partial oxidation process for the production of synthesis gas from a thixotropic slurry. The slurry comprises about 10 to 75 weight percent of particulate solid carboniferous fuels in water or in a hydrocarbon liquid fuel containing about 2 to 10 weight percent of soot produced, for example, by partial oxidation of crude oil. The partial oxidation of the slurries are accomplished using an annulus type burner wherein an oxidizing gas or a mixture of oxidizing gas and steam hits a relatively low velocity stream of the slurry, atomizing it and gasifying the atomized particles to form synthesis gas.
Example 1 of the patent discloses that the solid particles are mixed with the liquid fuel before the soot is added and Example 2 illustrates the mixing of all three components apparently simultaneously. There is no teaching or suggestion that a combustion enhancing material is absorbed in porous solid fuel.
The main purpose of the present invention is to produce heat developed when the compositions according to the present invention are substantially completely oxidized during burning in a standard type oil burner. The combustion products formed in the present invention include carbon dioxide and water vapor, rather than carbon monoxide and hydrogen. It appears that the compositions disclosed in the patent are gasified, whereas the compositions according to the present invention are burned.
U.S. Pat. No. 1,390,232 of Bates discloses a process for forming a liquid coal-oil slurry wherein the coal is pulverized in oil to promote fuel stability. Stability is further enhanced by addition of peptizing agents, such as coal derivatives or distillates or by the addition of protective colloids or soaps, such as lime-rosin soap. The coal particles tend to separate from the patented mixture (p. 2, lines 103-107). This patent defines "stable" as meaning "non-separation of the components to a reasonable extent and for a reasonable time sufficient to enable the use of the fuel by atomizing it as fuel oil is now used" (p. 2, lines 121-126). Thus, it is clear that the "stability" of the patented composition is not comparable to the stability of the present compositions wherein the solid particles are not suspended only to a reasonable extent and only for a reasonable time, but rather, are suspendable for many months, and even years. The stabilizing agents of this patent do not appear to form a thixotropic gel of the coal-oil slurry.
U.S. Pat. No. 1,647,471 of Plauson discloses a non-settling liquid suspension or emulsion of a solid combustible carbonaceous material in a liquid fuel. Optional emulsifying agents which accelerate emulsification include soap solutions, rubber solutions, or other colloidal material. In order to form the disclosed emulsion, it is essential to use a cross hammer mill to work the mixture for 1 to 2 hours. There is no recognition or suggestion that the emulsification or suspension is thixotropic or that combustion enhancing material is absorbed in porous carbonaceous material.
U.S. Pat. No. 2,668,757 of Hansley discloses dispersions of amorphous forms of elemental carbon in inert organic liquids. The dispersions are stabilized against settling by the incorporation of ingredients which form a soap in situ. It is stated that the soap may be prepared before, during or after the finely divided particles of carbon are added to the mixture. Thus, it is clear that it is not important for the purposes of this patent that any combustion enhancing materials be absorbed in the carbon particles.
U.S. Pat. No. 3,095,334 of Scurlock discloses a thixotropic gel fuel comprising a finely divided insoluble solid oxidizer in a non-volatile, substantially shock insensitive liquid fuel containing thixotropic gelling agents compatible with the non-volatile liquid fuel. Finely divided, solid metal powders may be added to the composition to increase density and improve specific impulse, which is particularly important when the fuel is a monopropellant. The solid combustible materials used in the composition of the present invention are not metal powders or oxidizers, but rather are carboniferous materials. The present composition is intended for use in standard oil burners which are adapted to use fuel without oxidizers within the fuel composition itself.
U.S. Pat. No. 3,210,168 of Morway discloses a stabilized slurry of pulverized coal coated with a liquid hydrocarbon fuel in water to make the slurry suitable for pumping through a pipeline. An imidazoline stabilizing agent stabilizes the slurry. Although the pulverized coal is coated with a liquid hydrocarbon fuel, there is no teaching or suggestion in the patent that the liquid hydrocarbon fuel is absorbed in the pores of the pulverized coal. In addition, there is no disclosure or suggestion that the patented slurries are combustible in standard type oil burners or that they are thixotropic.
U.S. Pat. No. 3,359,040 of Every et al. discloses a method of transporting particulate solid particles in a carrier liquid by pipeline. The solid particulate material, such as coal, is mixed with water which is adsorbed on the surface of the solid. The solid with the adsorbed water is then mixed with a hydrocarbon carrier liquid to form a slurry which may be stored without objectionable compaction. The slurry can be pumped through a pipeline without objectionable compaction within the pipeline. However, the solid particles settle when the slurry is allowed to stand.
It is clear from the patent that it is not related to a method of producing a thixotropic gel fuel wherein combustion enhancing materials are absorbed in porous, solid combustible material. The disclosure that water is adsorbed on the surface of coal particles and then mixed with a hydrocarbon carrier liquid does not teach or suggest the method of the present invention nor the compositions produced in accordance with the method. Thus, there is no indication that the patented slurry is thixotropic or that it is capable of being burned in a standard type oil burner. Nor is there any indication that the slurry is comprised of any of the gelling agents or ash modifiers set forth in the present application.
U.S. Pat. No. 3,449,178 of Tarpley, Jr. discloses thixotropic liquid oxidizer propellant mixtures comprising a mixture of a particular carbon black added to liquid oxidizer propellants. The gel has a yield stress of about 40 to 1500 dynes per square centimeter and may contain surfactants or solid materials which are non-reactive at storage temperatures. The mixtures are formed by shear working or ultrasonic dispersion. The patented compositions are not well suited for burning in standard type oil burners.
Other relevant patents which relate to coal in oil slurries, but which are not considered to be as relevant as the patents discussed above due to the different types of characteristics and stabilizing components, and different methods of making them, include: U.S. Pat. No. 2,397,859 of Hersberger et al., U.S. Pat. No. 2,754,267 of Biondi, U.S. Pat. No. 3,539,406, 3,613,372 and 3,617,095, all of Lissant, U.S. Pat. No. 3,732,084 of Nixon et al. and U.S. Pat. No. 4,030,894 of Marlin et al.